Section 3-4Section 3.4 Elements and Compounds

• Distinguish between elements and compounds.

• Describe the organization of elements in the periodic table.

• Explain how all compounds obey the laws of definite and multiple proportions.

proportion: the relation of one part to another or to the whole with respect to quantity

Section 3-4Section 3.4 Elements and Compounds (cont.)

A compound is a combination of two or more elements.

elementperiodic tablecompound

law of definite proportionspercent by masslaw of multiple proportions

Section 3-4Elements

• An element is a pure substance that cannot be separated into simpler substances by physical or chemical means.

• 92 elements occur naturally on Earth.• Each element has a unique name and a one, two,

or three-letter symbol.• The periodic table organizes the elements into a

grid of horizontal rows called periods and vertical columns called groups.

Section 3-4Compounds

• A compound is a made up of two or more elements combined chemically.

• Most of the matter in the universe exists as compounds.

• Table salt, NaCl, and water, H2O, are compounds.

Section 3-4Compounds (cont.)

• Elements can never be separated.

• Compounds can be broken into components by chemical means.

Section 3-4Compounds (cont.)

• This figure shows electrolysis of water to form hydrogen and oxygen.

Section 3-4Compounds (cont.)

• The properties of a compound are different from its component elements.

Section 3-4Law of Definite Proportions

• The law of definite proportions states that a compound is always composed of the same elements in the same proportion by mass, no matter how large or small the sample.

Section 3-4Law of Definite Proportions (cont.)

• The relative amounts are expressed as percent by mass, the ratio of the mass of each element to the total mass of the compound expressed as a percentage.

Section 3-4Law of Definite Proportions (cont.)

• This table demonstrates that the percentages of elements in sucrose remain the same despite differences in sample amount.

Section 3-4Law of Multiple Proportions

• The law of multiple proportions states that when different compounds are formed by a combination of the same elements, different masses of one element combine with the same relative mass of the other element in whole number ratios.

– H2O2 and H2O– Copper(I) chloride and copper(II) chloride

Section 3-4Law of Multiple Proportions (cont.)

Practice Problems: 76 (20-22)20. A 78.0 g sample of an unknown compound contains 14.4 g of hydrogen. What is the percent by mass of hydrogen in the compound?

% H = 12.9 x 100 = 15.9 % 78.0

Practice Problems: 76 (20-22)21. If 1.0 g of an hydrogen reacts completely with 19.0 g of fluorine, what is the percent by mass of hydrogen in the compound that is formed?

Compound Mass = 1.0 + 19.0 = 20.0 g % H = 1.0 x 100 = 5.0 % 20.0

Practice Problems: 76 (20-22)22. If 3.5 g X reacts with 10.5 g of Y to form the compound XY, what is the percent by mass of X in the compound? the percent by mass of Y?

Compound Mass = 3.5 + 10.5 = 14.0 g % X = 3.5x 100 = 25.0 % 14.0

% Y = 10.5 x 100 = 75.0 % 14.0